Photoacoustic sensors have been employed in the past for detection of gas species. Turning to FIG. 1, an example of a conventional photoacoustic sensor system 100 can be seen. This system 100 generally comprises a laser 102, optics 104, and an acoustic resonance chamber 106, tuning fork 108, lock-in amplifier 110, and function generator 112. In operation, the function generator 112 provides a drive signal to the laser 102 so as to modulate the beam emitted by the laser 102. The optics 104 can focus the beam along optical path 114 into the acoustic resonance chamber 106 (which contains a gas sample). By virtue of the photoacoustic effect, the modulated laser beam will cause the gas sample in the acoustic resonance chamber 106 to expand and relax if the wavelength of the laser matches the molecular resonance of the gas sample, which, in turn, causes the acoustic resonance chamber 106 to vibrate. Tuning fork 108 (which is generally placed in proximity to the acoustic resonance chamber 106 and which is generally a high-Q resonator) converts the vibrational signatures to electrical signals which is then amplified by the lock-in amplifier 110 (which also can receive the drive signal from the function generator 112). Based on the vibrational signatures, the identities and concentrations of gas species within the gas sample can be isolated.
This arrangement, however, does have some problems. For example, because this system 100, uses passive detection, the system 100 suffers from errors due to amplifier noise (i.e., used to amplify the signal from tuning fork 108) and ambient thermal noise as well as frequency drift and inaccuracy of tuning fork natural resonance. Therefore, there is a need for an improved photoacoustic sensor.
Some other conventional systems are: U.S. Pat. No. 4,184,768 U.S. Pat. No. 4,818,882; U.S. Pat. No. 5,479,259; U.S. Pat. No. 6,106,245; U.S. Pat. No. 7,245,380; U.S. Pat. No. 7,387,021; U.S. Pat. No. 7,520,158; U.S. Pat. No. 7,605,922; U.S. Pat. No. 7,797,983; U.S. Patent Pre-Grant Publ. No. 2008/0252891; U.S. Patent Pre-Grant Publ. No. 2009/0320561; U.S. Patent Pre-Grant Publ. No. 2010/0027012; and European Patent No. EP0685728.